Two years ago, an ultra-energetic event targeted Earth –

On May 27, 2021, an American telescope discovered the second highest energy cosmic rays in the history of their discovery. The energy intensity was so enormous that it challenges our ability to understand particle physics.

A recent publication in Science announces the discovery of a cosmic ray that fell to Earth on May 27, 2021, through a very special telescope built in the Utah desert.

The Earth is constantly bombarded with radiation from the cosmos, but this radiation is endowed with an energy never before observed, especially since this macroscopic energy is probably carried by a proton, that is, it is concentrated in an infinitesimal volume. There is no such energy density anywhere else on earth.

It is probably a tiny piece of dust that testifies to a catastrophic event that occurred very long ago in the depths of the sky, and whose origin poses a problem of interpretation for physicists.

The discovery of cosmic rays

In 1911, Victor Hess discovered radiation that comes from the cosmos. To this end, he did not hesitate to ascend in a balloon to an altitude of five kilometers to avoid the terrestrial radiation emitted by the radioactivity emanating from our planet. He used an “electroscope,” an instrument capable of measuring the flow of ionizing particles passing through it. He observed that the flow increased with altitude and therefore had its origin in space. Hess received the Nobel Prize in 1936.

The earth’s surface constantly receives around one hundred charged particles per square meter per second. These particles are “muons”, elementary particles similar to electrons, but with a higher mass.

But these particles are not, in themselves, cosmic rays coming from the depths of the cosmos: they are “secondary” particles created by interactions triggered in the atmosphere by protons or other heavy nuclei coming from much more distant areas . Upon arrival on Earth, only muons and neutrinos remain, as the other particles created have disappeared (either they decay or they interact alternately).

A shower of particles in the atmosphere

The atmosphere surrounding the Earth forms a thick skin several dozen kilometers thick. In total we have the equivalent of 10 meters of water above our heads. That’s a lot of matter and a proton arriving in the upper layers will inevitably interact with interactions during the crossing. On average, interaction with molecules in the atmosphere occurs at an altitude of about 20 kilometers.

The interactions between elementary particles are studied in detail in laboratory experiments such as those at CERN. Therefore, we know that a proton penetrating matter creates an initial interaction and, with increasing energy, creates a wider range of secondary objects: pions, kaons, etc. However, these particles will have the opportunity to interact in order, namely the so generated particles interact with each other. In the end we get what we call a “particle shower”.

We model the passage of protons in the atmosphere up to the energies reached at the accelerators and extrapolate for higher energies using computer simulation programs. The sheaf can extend for kilometers, with the heart being at an altitude of around 10 kilometers. The higher the energy of cosmic rays, the greater the number of secondary particles, and at the energies we are going to talk about, the shower can be rich in billions of secondary particles, covering several square kilometers of the Earth’s surface. The discovery of such showers allows us to return to the particle that produced them.

cosmic spraycosmic sprayDiagram of an atmospheric cascade created by a proton. // Source: Beetjedwars, Lacosmo, ComputerHotline

A gigantic telescope in the middle of the desert

How can we see how such showers form in the atmosphere? For Plato, knowledge is derived from interpreting the shadows perceived at the bottom of a cave. In the present case, the aim is to extract the properties of the cosmic rays responsible for the shower from the imprint left when it arrived on Earth.

Very high energy events are extremely rare. The one mentioned here has a reconstructed energy of 244 Exa-eV (244 x 1018 eV), and the corresponding flux is expected at the level of one copy per century and per square kilometer! Here, energies are measured in eV and their multiples, where 1 eV is the energy absorbed by an electron at a potential difference of 1 volt – a tiny energy equivalent to 1.6 x 10-19 joules in conventional units.

In order to be able to discover some of these rare phenomena, it is therefore necessary to build a huge telescope and to instrument as large a surface as possible.

The Telescope Array at the origin of this observation is located in a desert in Utah in the middle of the United States. It consists of a square network of 507 stations installed on the ground, each measuring 3 square meters, made up of “plastic scintillators” that respond to the passage of particles. The stations are distributed at a distance of 1.2 kilometers, resulting in a total sensitive area of ​​700 square kilometers. This terrestrial network is supported by skyward-facing fluorescence detectors: these are capable of detecting glowing trails associated with the showers that sweep the atmosphere on moonless nights.

The intensity of the signals collected provides information about the shower, allowing the energy of the responsible cosmic rays to be measured, and their direction of arrival is derived from the time differences measured at the different ground stations. The uncertainty is estimated at 1.5 degrees.

For furtherThe SKA Observatory.  // Source: SKA ObservatoryThe SKA Observatory.  // Source: SKA Observatory

The ultra-energetic event of May 27, 2021

Thus, the published event triggered a total of 23 simultaneous neighboring detectors in the telescope, covering an area of ​​about 30 square kilometers. A large proportion of muons is observed, which rules out that the original particle is a photon (photons produce electromagnetic showers composed of particles different from those expected for a proton) – however, a deeper examination of the composition of the bundle revealed There are no results to determine whether it is a pure proton or a heavier nucleus.

The reconstructed energy of 244 Exa-eV is subject to an uncertainty of about 25%. This is a colossal energy, 30 million times higher than the energy of the protons achieved by the accelerator at CERN that discovered the Higgs boson. This is equivalent to about 40 joules in current units. It is the energy that a tennis ball carries when it is hit by a champion during a major tournament. This is breathtaking energy on a macroscopic scale, concentrated in a particle – probably a proton – whose size does not exceed 10-15 meters!

The mystery of the origin of these cosmic rays

For Aristotle, the cosmos was unchanging – in contrast to the earth, which is transient. Cosmic rays, which the Greek philosopher could not have predicted, prove very clearly that the universe is in constant change. We now know that the sky holds gigantic dramas – black holes swallowing their neighboring stars, galaxies becoming telescopic, binary stars merging… We are a long way from the harmony we admire when we turn our gaze to the sky on a beautiful, star-studded summer night judge.

The publication cited describes an extraordinary event, the interpretation of which is not obvious.

At such energies, a proton cannot travel infinite distances in space because it is above the threshold for interaction with cosmic microwave background photons from the Big Bang. These photons, captured in particular by the Planck satellite, fill the entire space at about 400 per cubic centimeter and each carry a tiny energy of 10-4 eV. However, a proton of extreme energy has every chance of interacting with these photons and thus loses its original energy by transforming into other particles; This is called the Greisen-Zatsepin-Kuzmin limit (GZK). We can only receive such energetic rays when they come very close to us. This limit was clearly demonstrated thanks to a previous experiment in which the Auger Observatory covered 3,000 square kilometers in the middle of the Argentine pampas.

This means that in order to survive crossing the intergalactic medium, the beam under study must be generated less than 100 megaparsecs from Earth, that is, in our immediate neighborhood, barely 1% of the universe.

In total, the Telescope Array experiment has measured 28 showers with more than 100 exa-eV since 2008. Their distribution in the sky is isotropic, meaning they come from all directions. Therefore, we cannot clearly identify their source.

A group of galaxies.  // Source: Good Free Photos (cropped photo)A group of galaxies.  // Source: Good Free Photos (cropped photo)The true origin and causes of these cosmic rays are unknown. // Source: Good Free Photos (cropped photo)

For the record event of 244 Exa-eV, the direction of arrival points to a gap in the large-scale structure of the universe, which seems surprising a priori since no object that could have produced such a jet has been found in this direction.

Because the source particle is charged, perhaps unknown galactic or extragalactic magnetic fields bend the beam’s trajectory as it propagates, causing it to lose its original direction? The known fields are too weak.

The paper suggests another, bolder way out: such a beam, which appears to violate the GZK limit, could indicate a new effect that points to a flaw in our current knowledge of particle physics. This is the “new physics” that we use whenever a result deviates from the established path.

To move forward, we would have to significantly expand the current statistics, i.e. cover larger areas or wait an excessive amount of time. It would be more sensible if we could imagine new detection techniques. In fact, developments are underway to detect showers by the radio waves they emit, such as the GRAND project, or to observe them from space, such as the EUSO proposal.

The story is not finished.

The conversation logoThe conversation logo

François Vannucci, Professor Emeritus, researcher in particle physics, specialist in neutrinos, Paris Cité University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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